The invention relates to a ground-drilling device with a basic body and a drill head which is supported for movement in longitudinal axial direction relative to the basic body.
A ground-drilling device with a drill head which is supported for movement relative to a basic body is known for example from DE 195 08 542 A1. This is a self-propelled ground-drilling device, a so called ground rocket. This ground rocket has a basic body within which an impact piston is supported for movement in longitudinal axial direction. The impact piston is caused to cyclically move back and forth by the supply of compressed air, wherein in every cycle the impact piston impacts a front impact surface, whereby ultimately the kinetic energy of the impact piston is transferred to the basic body of the ground rocket to advance the latter in the soil. The front impact surface is formed by an impact bolt which is part of a drill head of the ground rocket. The drill head is supported in the basic body for movement in longitudinal axial direction; for guiding the relative movement between the drill head and the basic body the impact bolt is supported within a corresponding opening of the basic body. In addition, the impact bolt has a ring shaped shoulder at its section which extends into the basic body, with a diameter of the ring shaped shoulder being greater than the diameter of the opening into the basic body. This shoulder serves for transferring the energy which is initially transferred from the impact piston to the impact bolt or the drill head to the basic body after a defined relative movement between the drill head and the basic body. The advancement of the ground rocket in the ground occurs thus in two stages: first, the drill head is advanced by a defined distance in that the impact piston impacts the impact surface of the impact bolt; after a defined movement of the drill head relative to the basic body the ring shaped shoulder of the impact bolt impacts the impact surface of the basic body, whereby the residual energy is transferred to the basic body in order to cause the basic body to follow the drill head in the bore.
The ring shaped shoulder of the impact bolt together with the corresponding section of the basic body forms a free space whose size varies depending on the position of the drill head relative to the basic body. This free space is connected with the working chamber of the basic body via the annular gap between the ring shaped shoulder and the inner wall of the basic body, within which basic body the impact piston is cyclically moved. Although this annular gap is small it is not sealed for reasons of cost thus allowing (partial) pressure compensation between the working chamber and the free space, whereby pressurized air which was conducted into the working chamber, flows over into the free space. Because the pressure compensation between the free space and the working chamber occurs relatively slowly, an overpressure may be present in the free space after the ventilation of the working chamber, which overpressure impedes a return movement of the drill head into its retracted position. In order to avoid this, it is provided to connect the free space additionally to the environment so that an overpressure which forms in the free space can not only be released to the working chamber but also to the environment. In the ground rocket of DE 195 08 542 A1 this occurs via the also not sealed support of the impact bolt in the opening of the basic body.
The pressure compensation between the free space and the environment, is associated with the disadvantage that the entering of contaminations from outside into the free space through the non-sealed support of the impact bolt is even exacerbated; because the movement of the drill head relative to the housing into its front position first generates a negative pressure due to the speed with which this movement occurs, which negative pressure “draws” contaminations into the free space. These contaminations on one hand increase the wear of the movable components of the ground rocket and can lead to a decrease or loss of movability of the drill head.